Mistakes in study.com lessons/quizzes

[knaves]

Study.com is correct in this case.  Although a rhombus usually references an polygon with four equal sides that is not a square (and thus does not have 90 degree angles), the actual definition is  a quadrilateral with sides of equal length.  In simpler terms, a quadrilateral is four sided figure; a rhombus is a special type of quadrilateral with all four sides of equal length; a square is a special type of rhombus with all angles equal.